The present invention relates to the control of a vehicle. More specifically, the present invention relates to a process for preventing a vehicle from overturning around its longitudinal axis.
A process of this type is known from the German patent DE 196 02 879 C1, which is incorporated herein by reference.
This prior art process relates to recognizing the likelihood of a vehicle overturning, and in particular, a vehicle consisting of a tractor and a trailer. The danger of overturning is recognized through a minimal actuation of the trailer brakes, combined with the observation of the reaction of an ABS anti-lock braking system incorporated in the trailer. If the ABS starts regulating and carries out a locking prevention action in conjunction with this relatively weak braking, there is an imminent danger of the tractor-trailer overturning. Upon this situation occuring, either a warning signal is emitted, or a stronger brake intervention is carried out in order to reduce the transverse acceleration of the tractor-trailer.
In addition to recognizing the right time to start such a stabilizing braking intervention, it is also important to recognize the right time to terminate the braking intervention. That is, at the time when the danger of overturning is no longer acute, the braking intervention should be reduced to avoid braking the vehicle unnecessarily.
It is therefore an object of the present invention to disclose a simple and reliable process for preventing a vehicle from overturning around its longitudinal axis, in which a stabilizing braking intervention is terminated in a timely manner.
A process to prevent the overturning of a vehicle around its longitudinal axis, as e.g., when rounding a curve, automatically evaluates the potential danger of overturning, and then proceeds to apply braking as required, in the following manner:
a) applying a first braking force to at least one of the vehicle wheels on the outside of the curve, to prevent the vehicle from overturning;
b) applying a second braking force to at least one of the vehicle wheels on the inside of the curve, where the second braking force is weaker than the first braking force;
c) terminating the first and second braking forces when a rotational speed of a wheel on the inside of the curve accelerates in a predetermined characteristic manner.
The inventive process determines the potential danger of overturning by determining the transverse acceleration level of the vehicle, based on the rotational speeds of the wheels, and comparing this transverse acceleration level to a predetermined threshold level. When the transverse acceleration level of the vehicle exceeds the predetermined threshold level, a potential danger of overturning is indicated.
The process further evaluates the danger of overturning by checking a wheel on the inside of the curve to sense a characteristic reduction of the wheel""s rotational speed as a result of the second braking force. At the same time, the process ascertains whether a rotational speed of a wheel on the outside of the curve remains essentially unchanged.
Where applicable, the process also disables the anti-lock braking system slippage signals for a wheel on the inside of the curve when the second braking force is applied.
One advantage of the present invention is that the time when there is no longer a danger of overturning can be ascertained indirectly from the wheel load; i.e., by evaluating the behavior of the wheel with the lesser applied braking force. Using this technique, no further information need be obtained, such as the level of the center of gravity, or the actual transverse acceleration, which can only be ascertained by a sensor. Therefore, no additional sensors are needed, and the inventive process can be implemented very economically through a simple expansion of the control program of an electronic control device.
In the prior art processes to prevent overturning, in which the wheels on both sides of the vehicle are subjected to approximately the same, relatively high braking force, the wheels on the inside of a curve don""t show that slippage has decreased until the wheel load is relatively great. This results in a calculated value of transverse acceleration that is relatively high, which in turn results in a continuation of the high braking force. Such a relatively later termination of the ABS-regulation process is regarded as uncomfortable. If the vehicle is braked in this manner so that it almost comes to a stop, the further consequence may be endangering the surrounding traffic. In the present invention, however, a considerably lower braking force is applied to the wheels on the inside of the curve, which results in an earlier start-up of those wheels, and thereby to a lower calculated transverse acceleration level, which enables the braking intervention to be terminated earlier and more safely.
The present invention is applicable to conventional compressed-air-controlled braking systems for utility vehicles, as well as to braking systems using any other type of actuating energy, such as hydraulic compression or electrical servomotors.
Another advantage of the present invention is that the braking intervention to prevent overturning is terminated when the rotational speed of at least one wheel on the inside of the curve accelerates in a characteristic manner. This eliminates the need for a separate load sensor to recognize the load increase on the wheel on the inside of the curve. Instead, the rotational-speed sensor already included in an anti-locking system can also be used for this purpose. It is a further advantage of the present invention that the calculation for transverse acceleration requires no special programming to recognize that there is no longer a danger of overturning. Instead, the increasing rotational speed of the wheel on the inside of the curve is included in this calculation, which then shows a decrease of the transverse acceleration level as shall be explained in further detail below.
In the present invention, test braking with a relatively lower braking force is applied to at least one wheel on the inside of the curve, in order to recognize a potential danger of overturning. Also, the ABS slippage signals of at least one wheel subjected to the test braking are disabled. As a result, the wheel subjected to test braking is not influenced by the ABS. It is therefore not possible that the brake of the wheel subjected to the test braking is bled by the ABS due to excessive wheel slip, and it is not able to start up again. In contrast to a complete elimination of the ABS function, this suppression of the ABS slippage signals enables the anti-locking function to be maintained on the basis of acceleration signals, for as long as the wheel has contact with the ground. This helps to prevent damage to the tires, such as flat spots, for example.